Natural 15N abundances in coupled soil ecosystem nitrogen pools allow to determine nitrogen flux partitioning based on isotope fractionation modeling

Soil nitrogen processes have been investigated in detail at the process and the molecular-microbiological level, mostly using laboratory scale analyses, with recent research efforts focusing on quantifying the overall nitrogen turnover across spatial scales. However, challenges like methodological limitations, large spatial and temporal variability, and complex interacting control factors hinder accurate quantification and understanding of soil nitrogen turnover. While controls of nitrogen cycle processes are well established in the laboratory, applying these insights to field, regional and (cross)continental scales remains difficult and therefore also the validation of these processes and their controls in large-scale biogeochemical models due to the scarcity of in-situ data.

We here propose and demonstrate an isotope fractionation approach which is non-invasive (no addition of ¹⁵N labeled compounds in dissolved form) and allows to explore in-situ dynamics of soil  nitrogen cycling from the field scale to continental spatial patterns. The approach allows to determine the flux partitioning between the coupled pools of organic nitrogen in plants, soils and microbes, ammonium, nitrate, and gaseous nitrogen forms. Fluxes estimated include depolymerization, microbial uptake, mineralization, nitrification, and soil nitrogen losses. We present examples across a European climate, bedrock and land use transect on how to quantify (i) microbial nitrogen use efficiency, and (ii) fractions of inorganic nitrogen loss through hydrological or gaseous loss pathways (leaching of nitrate or gaseous losses via nitrification/denitrification in the form of NO, N₂O and N₂), based on isotope fractionation modeling of natural ¹⁵N abundance data of soil nitrogen pools.